1. Field of the Invention
The present invention relates to an optical element (mirror) making use of internal reflection, which is used in optical instruments (optical apparatus) such as cameras, copiers, printer exposure apparatuses and so on, and a method for producing the optical element.
2. Related Background Art
There have been proposals of optical members (plane mirrors and curved mirrors) of the internal reflection type in which light was incident to one surface of a transparent substrate and in which the light was reflected inside the substrate by a reflecting film formed on the other surface of the transparent substrate.
In general, ordinary mirrors of the internal reflection type used in optical instruments are those obtained by forming a reflecting film of metal such as Al (aluminum), Ag (silver), or the like having high reflectances in the visible light region, on a substrate and then laying a protective film of a dielectric film or a metal film thereon or a protective film of a coating film thereon.
The internal reflection type mirrors of Al according to the conventional technology, however, were apt to have low reflectances throughout the entire wavelength range and particularly in the long wavelength region. Further, the Ag films had relatively high spectral reflectances, but were apt to decrease the reflectances in the short wavelength region when subjected to an ion bombardment cleaning operation or the like for enhancing adhesion. A photographing optical system using a lot of such internal reflection type mirrors had a problem that transmitted light thereby was light of colored spectral characteristics.
Also, the protective film of the coating film required a new device or jig separate from an evaporation system used for formation of the metal film. This posed problems that the production steps became complex and that the cost of the production apparatus was high.